Virtually all modem passenger automobiles and most modern trucks employ disc brake systems on at least the front wheels. When the driver depresses the brake pedal, a hydraulic system forces brake pads against each side of a disc-shaped, metallic brake rotor. This rotor is attached to the automobile at the hub, which is connected to the suspension system. The respective axes around which the hub and rotor rotate must be precisely perpendicular to their respective mating surfaces and also aligned with each other in order to maximize the efficiency and life of the braking system. If the axis of rotation of the hub and the axis of rotation of the rotor are not precisely aligned with each other or are not precisely perpendicular to their respective mating surfaces, then the surface of the rotor will oscillate laterally as the rotor turns and cause the pressure between the rotor and the brake pads to vary, sometimes dramatically. Drivers sometimes, in certain vehicles, sense the runout in the form of a pulsating brake pedal. Because the braking system is a discrete subsystem within the overall wheel assembly, misalignment within the braking system as described above cannot be corrected using conventional devices for correcting misalignment in other portions of the wheel assembly (e.g., devices for correcting camber).
Such misalignment is caused by any combination of factors including: inaccuracy in hub geometry, imperfections in the hub and rotor mounting surfaces, inaccuracy in the bearings that are mounted in the hub mating surfaces, inaccuracy in rotor geometry, improper surface finishing of the rotor or the hub, and worn or inaccurate refinishing equipment (e.g., bench lathes). More important than causing an anomalous sensation in the brake pedal, however, this misalignment reduces braking efficiency and accelerates the rate at which the surfaces of the rotor and the brake pads wear. The misalignment also causes uneven heating in the rotor that leads to uneven wear and warping of the rotor. Excessive warping of the rotor exacerbates the pulsing problem sensed through the brake pedal and requires that the brake technician remove an excessive amount of material from the rotor when resurfacing it.
Brake technicians typically measure such misalignment using special tools. For example, they may rotate the rotor in the presence of a stationary stylus that is attached to a micrometer and which follows the rotor surface. The micrometer records the maximum lateral deflection of the brake rotor as the rotor rotates. This maximum lateral deflection is known as "runout" and is expressed as a linear dimension. Most manufacturers specify that runout for a late model automobile should be no more than approximately 0.002-0.003 inches, but often, runout of as much as 0.012 inches is encountered by brake technicians.
Conventional methods of correcting rotor runout employ a lathe to resurface a brake rotor by removing a very thin layer from the outer surface of the rotor. Typically, rotors are resurfaced using a bench lathe, which requires that the rotor be removed from the hub before resurfacing, or a portable lathe mounted directly to the rotor while it remains attached to the hub and the automobile. Bench lathes currently are the best tool for creating a superior surface finish on a brake rotor. However, bench lathes sometimes fail to compensate fully for many causes of runout, which are only evident when the rotor is mounted to the hub. By contrast, an on-the-car lathe is sometimes effective at correcting runout, but it often must remove an inordinate amount of material from the rotor to do so. However, typical on-the-car lathes do not finish the surface of the rotor as well as a bench lathe.
In order to ensure that the brake rotors are able to dissipate the heat generated by braking, manufacturers currently specify that a total of no more than approximately 0.030 to 0.070 inches of material may be removed from the surface of the brake rotor. If too much material must be removed for an on-the-car lathe to correct the runout measured by a technician, then the rotor must be replaced. Thus, methods employing on-the-car lathes can "waste" rotor material and can significantly shorten the life of the rotor. For example, when correcting for 0.006 inches of runout, a brake technician must adjust an on-the-car lathe to remove at least 0.006 inches per side of the rotor. Thus, at least 0.012 inches of material must be removed, which is a significant fraction of the total amount of material that can ever be removed from the rotor. Such excessive removal of material drastically reduces the overall life of the rotor and decreases the ability of the rotor to dissipate heat, which may accelerate the rate at which the rotor warps and thus shorten the time between required rotor servicing.
Another conventional method of correcting rotor runout is described in U.S. Pat. No. 5,108,156 ("the '156 patent"), which is incorporated into this document in its entirety by this reference. The '156 patent describes a set of relatively thick washers of a single thickness held in a fixed circumferential relationship by a thinner ring-shaped body (also of uniform thickness). According to the '156 patent, its ring-and-washer set is interposed between the hub and rotor and purportedly corrects runout by accommodating "surface imperfections, including dirt and rust particles between the hub . . . in a space defined by the washers."Additionally, the shim corrects runout by "minimizing! the surface area of contact between the hub and rotor."
However, the ring-and-washer set disclosed in the '156 patent cannot correct for: any misalignment between the respective axes of rotation of the hub and rotor, a lack of a perpendicular relationship between the axis of rotation and the mating surfaces of the hub and rotor, or surface imperfections in hub and rotor that lie under the washers near the lugs. Moreover, significantly reducing the contact area between the hub and rotor may cause long term wear of the hub and rotor due to localized stress concentration where the washers rest upon the hub and rotor. Because of the problems associated with existing hardware and methods for correcting brake rotor runout, a need continues to exist for an efficient, inexpensive, and effective method of correcting brake rotor runout.